1. Field of the Invention
The invention relates to ion mobility spectrometers that can be used to detect the presences of minuet amounts of substances of interest.
2. Description of the Related Art
Previous ion mobility spectrometers and ion trap mobility spectrometers are capable of uniquely identifying approximately 100 peaks in the drift spectrum. For many applications in drug and explosive trace detection this provides sufficient resolution to identify the target materials while maintaining very low false positive responses. This performance level is largely achieved by the use of dopants to eliminate many unwanted ionic species from innocent materials, thus producing simpler spectra with fewer xe2x80x9cpeaksxe2x80x9d. An example of an effective use of dopants is disclosed in U.S. Pat. No. 5,491,337.
More recent improvements have been developed in which both positive and negative ion spectra are produced from the same sample. Such improvements are disclosed in U.S. patent application Ser. No. 10/103,601. This allows improved resolution for materials that produce both negative and positive ion spectra. It also allows both drugs and explosives to be detected in the same sample.
Yet another improvement allows the reaction rate chemistry of the ion transfer reactions to be measured prior to drifting ions down the drift tube, as disclosed in U.S. patent application Ser. No. 09/910,197. This provides increased detection capability and resolution by recording the ionic spectrum or plasmagram at selected time intervals after injecting charge into the reaction chamber of the spectrometer.
The above methods and improvements have allowed superb detection and identification of most drugs and explosives. However, some ionic species remain difficult to identify separately.
The dopants added to the carrier flow entering the detector scavenge the charge from the weaker charge affinity materials. Thus, there is also a need to detect and identify a wide range of materials that would not normally be ionized.
The present invention provides for further increases in resolution (ability to determine the difference between two similar ions) and an increase in the range of materials that can be identified uniquely.
The present invention is a method and apparatus which modifies the design of either an ion mobility spectrometer (IMS) or an ion trap mobility spectrometer (ITMS) to provide improved resolution between coincidental or closely spaced peaks in the mobility spectrum. For example, the peaks from tetrahydro cannabinol (active ingredient in marijuana) and heroin are closely spaced. These two ions normally can be separated with some care to maintain all conditions constant, but occasionally the heroin contains other opiates such as papavarene, which makes the separation extremely difficult.
The time of flight of a specific ion is decided by the length of the drift tube, the electric field strength down the drift tube, and the mass and shape of the ion. The shape factor is difficult to predict, but generally, a more spherical ion will normally take a shorter time than an ion of the same mass which is more elongate. As the field strength is increased, ions which exhibit a dipole moment may have more tendency to align with the field. This means that they may xe2x80x9ctumblexe2x80x9d less in a high field than in a low field. Thus the shape and polarity factor may cause two ions which have identical drift times at one field strength to have differing drift times at a different field strength. This parameter has been used in asymmetric field effect ion mobility detectors to provide a detection capability. The performance of such detectors has been inferior to traditional ion mobility detectors because only one ionic species can be measured at one time, and scanning through the range of differential mobilities is relatively slow.
The present invention takes advantage of the performance of the IMS and ITMS with the additional advantage of the asymmetric field effect ion mobility spectrometer. After a substance is detected and tentatively identified, the field strength is reduced electrically by reducing the voltage supplied to the field defining electrodes in the drift tube. This reduces the mobility of all ions, but not all are changed in proportion to the original drift time at high field strength. The peak shift between the two ion spectra is measured and compared against standard shifts held in a library or storage to confirm the identity of the material. If no match is obtained then the substance is unknown and is not identified. Thus false alarms from unknown materials are eliminated.